Calculating fuel economy and usage

Manufacturers quote a figure that indicates how fuel-efficient their cars are. Usually this is expressed in miles per gallon, but on some Japanese imports you might see litres per 100km or kilometres per litre. The figures are given as three measurements: urban (i.e. around town), extra urban (open roads) and combined (a mixture of the two. The tests are standardised so, while they are woefully inaccurate in the real world, they at least provide a consistent benchmark and a means of comparing one car with another.

For example, if two cars both have figures of 30 miles per gallon you know they are roughly equal, but whether you get 30mpg or 50mpg depends on your driving style, how many passengers you carry, whether you live in a hilly area, what kind of fuel you use, what altitude you live at, whether you tow a trailer, and how well you maintain your car. It all gets more complex if you have a hybrid car.

Quoted fuel economy for plug-in hybrids

If you have a plug-in hybrid such as an Audi A3 e-tron, which has a petrol and electric motor whereby you can plug the electric motor in the wall, then the quoted figure will be less than the absolute figure because it depends on your driving. If the average trip distance falls within the average range of the batteries then you wouldn’t technically use any petrol as you will just be charging the car overnight. In these cases you might see a figure of 2l/100km.

Here’s an example (hypothetical) of how it works:

If the electric motor range is 30 miles and the tested distance is 60 miles, then the first 30 miles consume no petrol. If the next 30 miles consume a gallon of petrol, then the quoted figure would be calculated on travelling 60 miles and using one gallon, i.e. 60mpg. In reality, the petrol engine is 30mpg, which means that if your journeys are much longer than the electric motor range, you will get much less real-world fuel economy. The average distance travelled per day by commuters is under 15 miles, so would fall within the range of an electric motor, which you would simply plug in at night and charge from the mains. But if you lived in a rural area and it was 20 miles either way you would end up using at least 10 miles of petrol every day, even if you charged the car each night. Drivers also have to remember to charge the car each night, too.

How to calculate realistic fuel economy for your car

Miles per gallon

Divide the miles driven by the amount of gallons it took to refill the tank. For example, you travelled 400 miles on 20 gallons, therefore 400 / 20 is 20 miles per gallon, or 20mpg.

If you do this every time you fill your petrol tank then you can take the average of a number of measurements which will give you a more realistic figure than a one-off measurement. Remember that if you are measuring your fuel then you will consciously or subconsciously change your behaviour because you have started a competition with yourself, and over time you will forget that you are doing it, and your fuel economy will slip.

Wait until your tank gets to 1/4 full before you fill it each time. If you fill at various points in the tank then you introduce a variable into the experiment: your car will be more economical when the tank is almost empty because it will be lighter.

It can help keep consistency if you fill up at the same place or at lease with the same brand of fuel while you are doing this test.

Gallons per 100 miles

You’re more likely to see litres per 100km, but you can use the same equation.

Divide the number of gallons it took to refill the tank by the distance travelled and multiply it by 100. E.g. 30 gallons / 600 miles x 100 = 5 gallons per 100 miles.

Putting this figure back into the first equation to find miles per gallon you would get 100 miles / 5 gallons = 20mpg.

You can search your car in this database then see if your actual real-world driving figure is anywhere near.

Saving money on fuel

We wrote an article with heaps of tips on saving fuel.